(a) Field of the Invention
The present invention relates to a mode converter, and more particularly, to a single-to-multi mode converter that converts a single mode to a multi-mode and a code division multiple access system using the same.
(b) Description of the Related Art
A mode converter is a device that converts modes used in optical communication networks. U.S. Pat. No. 5,077,815 discloses a mode converter, which directly connects a single-mode optical fiber to a multi-mode optical fiber. The mode converter converts a single-mode to a multi-mode through core axial displacement between the single-mode optical fiber and the multi-mode optical fiber. With this mode converter, however, it is very difficult to accurately align the axes of optical fibers with each other according to a designed value to connect the optical fibers to each other. Furthermore, a field distribution of a fundamental mode of the single-mode optical fiber is not suitable for uniformly coupling the power of a fundamental mode of the single-mode optical fiber to each mode of the multi-mode optical fiber during the mode conversion process.
To solve the aforementioned problems, there has been proposed a method that cuts optical fibers that will be connected to each other such that their cross-sections have a specific angle, and then connects the optical fibers to each other. However, the cutting and connecting processes are difficult to carry out. Moreover, it is not easy to decrease errors in the processes.
S. Choi et al. proposed a mode converter using an optical fiber, which is disclosed in an article entitled “Novel mode converter based on hollow optical fiber for Gigabit LAN communication”, IEEE Photonics Technology Letters, Vol. 14, No. 2, pp. 248 to 250, 1202. This method connects a tapered hollow optical fiber whose core thickness is gradually increased between a single-mode optical fiber and a multi-mode optical fiber to convert a single mode to a multi-mode. However, this technique couples the power of a fundamental mode of the single-mode fiber to a higher-order mode group having relatively small modal dispersion in order to obtain small modal dispersion. Thus, it is difficult to uniformly couple the power of the fundamental mode of single mode fiber to each mode of multi-mode fiber.
In an optical communication network or an optical access network, excess noise or speckle noise caused by a beat reduces a signal-to-noise ratio to decrease transmission quality and restrict the number of subscribers.
In particular, in the case of an optical access network using code division multiple access, the excess noise is an essential factor that restricts the number of subscribers because the excess noise is in proportion to the square of the number of subscribers. Accordingly, the optical access network requires a technique capable of reducing the excess noise to maximize the number of subscribers.
Noises generated in optical communications include an optical intensity noise that is generated in a laser diode of a transmitter, and a shot noise and a thermal noise generated in a receiver.
In a subcarrier multiplexing optical access network or an optical code-division multiple-access optical access network, signals transmitted from respective subscribers are added up so that an optical signal with a high intensity is received. This generates the excess noise. Particularly, in a spectrally encoded optical CDMA network using a broadband incoherent light source such as a superluminescent light emitting diode as a light source, a large excess noise is generated which restricts the number of subscribers. Furthermore, the performance of an optical fiber gyroscope using the broadband incoherent light source is restricted by the excess noise.
A method of reducing the excess noise caused by a beat is disclosed in U.S. Pat. No. 5,978,528. This method splits a light beam emitted from a light source into a plurality of components using a splitter. Each component travels in a separate path having a unique time delay which is longer than the coherence time of the light source. The lights from separate paths are recombined through a recombiner to be input to a photodetector.
With this technique, the excess noise is reduced as the number of split components is increased. However, an optical loss in the splitter and recombiner is very large during the splitting and recombining process, respectively. Furthermore, this technique should maintain polarization. Moreover, since the split components of the beam should travel through optical fibers of different paths to be recombined, a large number of optical fibers are required to considerably reduce the excess noise.
Furthermore, since the split components that have traveled in separate optical fibers should be recombined with different time delays that are longer than the coherence time, the optical fibers should have different lengths.
Moreover, the method of simply connecting a single-mode optical fiber to a multi-mode optical fiber has very low mode conversion efficiency. In addition, the multi-mode optical fiber must be very long in order to allow adjacent modes to reach the photodetector with different time delays of longer than the coherence time.